When digital data is transmitted from one location to another over a communications channel, it is necessary for both the transmitting and receiving stations to operate according to an established message format. The message format is a function of many factors including data rate, expected error rate and error checking, word size of the data, type of communications channel and configuration of the network used. In actual practice it is desirable to utilize a widely accepted standard format, if possible, in order to enable communications with a large number of terminals and to reduce the time which would be required to develop a new format.
One of the new and widely accepted data communication formats, or protocols as they are often called, is Synchronous Data Link Control (SDLC). SDLC allows a great deal of flexibility in message protocol with respect to previously used protocols, such as Binary Synchronous Communication (BSC). SDLC is a bit oriented protocol in which any size data word may be transmitted, whereas BSC is byte oriented, requiring data to be transmitted in multiples of eight bits; also, SDLC has very flexible provisions for message switching and error control.
Prior data communication control equipment has been inflexible because of the inflexible protocols used. A data communications adapter can be built for SDLC using the same inflexible techniques as previously used if only one particular use of SDLC is considered; however, there is now a need for a flexible data communication control arrangement which is capable of taking advantage of all the features of SDLC which might be used in the future.
The present invention results in a very flexible data communications adapter which may be programmed by the Common Control processor to which it is connected, thereby affording a large number of the available variations of SDLC formats. The invention is very efficient in that it operates independently once it has been initialized, transmitting or receiving a bit serial data stream according to the specified format. During both transmission and reception, data coupling with the process takes place using Direct Memory Access (DMA).